I. Field of the Invention
The present invention relates generally to the field of cardiac rhythm management devices, including atrial, ventricular, and dual chamber pacemakers. More specifically, the present invention relates to a portion of the sensing circuit utilized during capture verification. The sense amplifier of the present invention includes a dedicated wide-band pass filter and reduces both the amplitude of the pacing artifact and the required recovery time, without requiring unipolar sensing or low impedance leads. The cardiac rhythm management device utilizes an adaptive evoked response sensing threshold and may operate in a normal beat to beat pacing mode and/or an autothreshold mode.
II. Discussion of the Prior Art
For the most part, prior art implantable cardiac rhythm management devices, including bradycardia and tachycardia pacemakers and cardiac defibrillators, have sense amplifier circuits for amplifying and filtering electrogram signals sensed through electrodes placed in or on the heart and which are coupled by suitable leads to the implantable cardiac rhythm management device. The signals emanating from the sense amplifier are applied to one input of a comparator circuit whose other input is connected to a reference potential. Only when an electrogram signal from the sense amplifier exceeds the reference potential threshold will it be treated as a cardiac paced or sensed beat. The source reference potential may be referred to as a sensing threshold. Only when an electrogram signal from the sense amplifier exceeds the preprogrammed reference potential threshold will it be treated as a cardiac paced or sensed beat. The source referenced potential may be referred to as an evoked response detection threshold.
For cardiac pacer systems having beat-to-beat capture verification and backup pacing it is necessary to detect the evoked response during the QRS complex. Pacing artifacts and the required recovery response period make it difficult to detect capture during the interval coinciding with the QRS complex. Thus, there is a need for a sense amplifier that assists in the reduction of pacing artifact and required recovery response period.
Typically, the reference potential threshold is set at a fixed amount that is expected to exceed the maximum amplitude of artifact. If, however, the threshold is not set high enough, then artifact may result in false capture declaration. Further, if the reference potential threshold is set too high, then the amplitude associated with an R-wave may not be sufficient to trigger the reference potential threshold.
Several factors influence the amplitude associated with R-waves. For example, respiration of the patient fluctuates the amplitude of the evoked response in a cyclic fashion increasing and decreasing over several beats. This fluctuation makes it even less desirable for the reference potential threshold to be set at a fixed amount. Other factors, described in greater detail below, affect the amplitude of the evoked response. Thus, a need exists for a cardiac rhythm management device that automatically adjusts the evoked response detection threshold in conjunction with fluctuations in the amplitude associated with a particular R-wave, taking into account the modulation of the amplitude corresponding to the evoked response for each R-wave.
U.S. Pat. No. 5,161,529 issued to Stotts et al. (the ""529 patent) describes a sense amplifier having a switched capacitor circuit, wherein the amplifier""s bandpass frequency characteristics are switched during a cardiac cycle to selectively vary the cardiac signal frequencies subject to sensing. In this manner Stotts et al. describes switching the bandpass frequency of the sense amplifier to a high bandpass frequency to detect intrinsic cardiac response and, within a suitable delay interval (10-30 ms) from delivery of a pacing stimulus, to a low bandpass frequency to detect an evoked response. Although Stotts et al. further describes optionally increasing the delay interval to an even higher bandpass frequency to assure rapid attenuation of any pacing artifact, the suitable delay interval was identified as ranging between 10-30 ms. Thus, there is a need for a sense amplifier that reduces both the amplitude of the pacing artifact and the required recovery time.
The success of a cardiac rhythm management device in causing a depolarization or evoking a response hinges on whether the energy of the pacing stimulus as delivered to the myocardium exceeds a threshold value. This threshold value, referred to as the capture threshold, represents the amount of electrical energy required to alter the permeability of the myocardial cells to thereby initiate cell depolarization. If the energy of the pacing stimulus does not exceed the capture threshold, then the permeability of the myocardial cells will not be altered and thus no depolarization will result. If, on the other hand, the energy of the pacing stimulus exceeds the capture threshold, then the permeability of the myocardial cells will be altered such that depolarization will result.
Changes in the capture threshold may be detected by monitoring the efficacy of stimulating pulses at a given energy level. If capture does not occur at a particular stimulation energy level which previously was adequate to effect capture, then it can be surmised that the capture threshold has increased and that the stimulation energy should be increased. On the other hand, if capture occurs consistently at a particular stimulation energy level over a relatively large number of successive stimulation cycles, then it is possible that the capture threshold has decreased such that the stimulation energy is being delivered at a level higher than necessary to effect capture. This can be verified by lowering the stimulation energy level and monitoring for loss of capture at the new energy level.
The ability to detect capture in a cardiac rhythm management device is extremely desirable in that delivering stimulation pulses having energy far in excess of the patient""s capture threshold is wasteful of the cardiac rhythm management device""s limited power supply. In order to minimize current drain on the power supply, it is desirable to automatically adjust the cardiac rhythm management device such that the amount of stimulation energy delivered to the myocardium is maintained at the lowest level that will reliably capture the heart. To accomplish this, a process known as xe2x80x9ccapture verificationxe2x80x9d must be performed wherein the cardiac rhythm management device monitors to determine whether an evoked response or R-wave occurs in the heart following the delivery of each pacing stimulus pulse.
For the most part, prior art implantable cardiac rhythm management devices, including bradycardia and tachycardia pacemakers and cardiac defibrillators, have sense amplifier circuits for amplifying and filtering electrogram signals sensed through electrodes placed in or on the heart and which are coupled by suitable leads to the implantable cardiac rhythm management device. The signals emanating from the sense amplifier are applied to one input of a comparator circuit whose other input is connected to a reference potential. Only when an electrogram signal from the sense amplifier exceeds the reference potential threshold will it be treated as an evoked response. The source reference potential may be referred to as a sensing threshold. In some instances the amplitude of pacing artifact may be so great that it becomes difficult to distinguish the amplitude corresponding to an evoked response with the amplitude corresponding to artifact. Hence, there is a need for a capture verification circuit of a cardiac rhythm management device capable of differentiating between the amplitude corresponding to evoked response and the amplitude corresponding to artifact of a sensed signal. There is a further need for a capture verification circuit suitable for use with either unipolar or bipolar stimulation and which does not depend upon lead placement. The present invention meets these and other needs that will become apparent from a review of the description of the present invention.
It is accordingly the objective of the present invention to provide for a cardiac rhythm management device having a capture verification sensing circuit which reduces both the amplitude of the pacing artifact and the required recovery time, without requiring unipolar sensing or low impedance leads. The cardiac rhythm management device of the present invention may operate in a normal beat to beat stimulation mode and/or an autothreshold mode and may be electrically coupled to one or more known suitable leads having pacing/sensing electrodes coupled thereto. The electrodes of each lead are selectively positioned within the heart and are coupled to an electrical conducting means for electrically coupling the electrodes to the cardiac pacer. Without any limitation intended, the electrical conductors and/or electrical conducting means as identified herein may comprise a circuitry having a plurality of electrical conducting segments of known suitable construction.
The cardiac rhythm management device has means for both selectively delivering electrical stimuli to a patient""s heart and for detecting at least one of intrinsic and paced stimulations. For ease of discussion and without any limitation intended the sensing circuit of the present invention will be described incorporated into a cardiac pacer. The cardiac pacer includes a sensing amplifier electrically coupled to the electrodes, wherein the sense amplifier amplifies and filters electrocardiogram signals collected by the electrodes. A dedicated evoked response sense amplifier is included having a wide-band pass filter. Also included is a power supply, controller coupled to receive the sensed electrogram signals and means controlled by the controller for applying cardiac stimulation pulses to a patient""s heart, wherein the stimulation pulses are applied in response to control signals from the controller.
The controller may be in any of several forms including a dedicated state device or a microprocessor with code, and may include ROM memory for storing programs to be executed by the controller and RAM memory for storing operands used in carrying out the computations by the controller. Those skilled in the art will appreciate that pacing circuitry, sensing circuitry, timing circuitry, and wave detection circuitry among others may all be included within the controller. The controller and components contained therein or coupled thereto detect and distinguish cardiac depolarization deflections and noise deflections from the electrocardiogram signal. A peak detector, for example, may be utilized to determine the amplitudes of the cardiac depolarization deflections and noise deflections. The sense amplifier of the present invention may be utilized during a capture verification mode of the cardiac pacer to reduce both the amplitude of the pacing artifact and the required recovery time of the pacing/sensing circuit.
The sense amplifier is formed as a part of the sensing circuit and includes the following components electrically coupled together via an electrical conductor: a preamplifier, a first high pass coupling capacitor, and a blanking switch. The high pass coupling capacitor is electrically coupled between the electrodes and the pre-amplifier. Further, the blanking switch is electrically coupled between the high pass coupling capacitor and the pre-amplifier. Electrically coupling the high pass coupling capacitor between the electrodes and the blanking switch reduces the affects of polarization voltages or afterpotentials. Additionally, a high pass termination resistor may be coupled to the electrical conductor between the blanking switch and the pre-amplifier.
Also, a low pass coupling capacitor and low pass resistor may be connected to the electrical conductor between the blanking switch and the pre-amplifier, wherein a low pass bi-pass switch may be connected to the electrical conducting means between the blanking switch and the pre-amplifier to selectively connect the low pass coupling capacitor and the low pass resistor to the electrical conductor. Further, an input blanking means may be connected to the electrical conductor between the blanking switch and the pre-amplifier for selectively blanking sensed electrical activity. Those skilled in the art will appreciate that a second sensing means for sensing electrical activity of the patient""s heart may be connected in parallel with the first sensing means as described above, thereby creating a differential network to offset imbalance sensed from the electrode due to extraneous factors.
It is accordingly a principal object of the present invention to provide a capture verification sensing circuit suitable for use in unipolar or bipolar sensing, wherein the leads may have a wide range of impedances.
Another object of the present invention is to provide a cardiac pacer that shortens the pacing afterpotentials and reduces sensing circuit reaction to residual charges.
A further object of the present invention is to provide a cardiac pacer having a capture verification sensing circuit which reduces both the amplitude of the pacing artifact and the required recovery time, without requiring unipolar sensing or low impedance leads.
These and other objects and advantages of the present invention will be readily apparent to those skilled in the art from a review of the following detailed description of the preferred embodiment in conjunction with the accompanying claims and drawings in which like numerals in the several views refer to corresponding parts.